The present invention relates to End of Life detection and safety circuits and in particular to temperature independent End of Lamp Life safety circuits.
Low-pressure discharge lamps are driven by electronic ballasts that deliver preheating voltage for the lamp filaments, ignition voltage to ignite the lamps, and the lamp drive current in the steady state. Further, the electronic ballasts use a Power Factor Correction Stage to control the harmonic content of the input current. Moreover, the standards for electronic ballast manufacturing require inclusion of safety circuits for the detection of lamp End of Life (EOL).
FIG. 1 illustrates a prior art EOL safety circuit 10 having a window comparator that includes comparators 12 and 14. The comparator 12 receiving 3V reference voltage at its positive terminal and the comparator 14 receiving 1V reference voltage at its negative terminal, a negative terminal of the comparator 12 and a positive terminal of comparator 14 are coupled to a cathode terminal of a Zener diode DEOL1, and output terminals of the comparators 12 and 14 are coupled to provide an EOL signal. The EOL safety circuit 10 further includes resistors RL1, RL2 series coupled at a node V1, a capacitor CAV parallel coupled to the resistor RL2, a Zener diode DEOL2 having a cathode terminal coupled to a node V1 and an anode terminal coupled to an anode terminal of the Zener diode DEOL1. The circuit 10 also includes an amplifier 16 and a resistor R1.
At EOL the lamps develop a DC-Offset in the lamp-voltage. This offset is sensed by the resistors RL1, RL2, and the capacitor CAV. The circuit 10 detects EOL if voltage at node V1 increases or decreases to a level that Zener diodes DEOL2 or DEOL1 start conducting and the voltage on the input of the window comparator goes above 3V or below 1V. When the voltage goes outside this window, EOL NOT goes low, signifying end of life. The drawback of circuit 10 is that two Zener diodes are required for EOL detection. The Zener diodes have temperature dependency and a low accuracy at low currents.